1. Field of the Invention
The present invention relates to a process for producing methacrylic acid from isobutylene. More particularly, it relates to a novel process for producing methacrylic acid from isobutylene by a two step oxidation consisting of a first step of oxidizing isobutylene to methacrolein and a second step of oxidizing methacrolein to methacrylic acid.
2. Description of the Prior Art
Heretofore, it has been known to produce methacrylic acid by reacting isobutylene with molecular oxygen in the presence of a diluent gas and steam. However, a catalyst suitable for industrial production of methacrylic acid from isobutylene in high yield, has not been found. Accordingly, it is industrially advantageous to conduct the reaction in two steps using a desirable catalyst for production of methacrolein from isobutylene and a desirable catalyst for production of methacrylic acid from methacrolein.
However, when methacrylic acid is produced in such a two step oxidation, it is relatively difficult to attain high selectivity of methacylic acid in the second oxidation step if the conversion of methacrolein is high. That is, the selectivity of methacrylic acid formation decreases with increasing conversion of methacrolein. Accordingly, it becomes advantgeous to conduct the second oxidation step under conditions such that the conversion of methacrolein is relatively low and then to recover the unreacted methacrolein from the reaction products and recycle them back into the second oxidation step. The unreacted methacrolein is separated from the reaction products in the second oxidation step while the object product, methacrylic acid is recovered. This is accomplished by cooling the reaction products in the second oxidation step to liquefy them, and/or further contacting them with water to separate the gaseous components of oxygen, nitrogen, carbon dioxide gas and the like. As a result, a liquid phase containing methacrolein and/or methacrylic acid, is obtained and these are separated by distillation.
However, the present inventors have determined that in many cases a dangerous self-explosive gas is formed in the process of recovering the products of the second oxidation step. The self-explosive gas includes self-flammable gas, usually called a detonating gas. The present inventors have further determined that the explosive gas is apparently formed by the following mechanism. The methacrolein in the reaction product has a relatively low boiling point and a low solubility in water. As a result, it is relatively difficult to recover a high proportion of the methacrolein in the liquid phase during the recovery process, because it is so insoluble. However, the methacrylic acid and steam in the reaction product can be easily recovered in the liquid phase. Accordingly, during the recovery process, methacrolein remains present in the gaseous system of oxygen, nitrogen and carbon dioxide gas of the reaction product.
Furthermore, the conversion of methacrolein in the second oxidation step is relatively low because of the characteristics of the catalyst. The consumption of oxygen is correspondingly small and a large amount of oxygen remains in the reaction product. Accordingly, the overall composition of the methacrolein-containing gas formed in the recovery process becomes explosive. In order to prevent this formation of an explosive gas, decreasing the supply of oxygen while increasing the supply of nitrogen used as a diluent gas has been suggested. Unfortunately, in this method, the resultant conditions are far from those which are optimum for catalyst performance in the second oxidation step, whereby the yield of the object product in the second oxidation step is disadvantageously decreased. Consequently, there remains a need for an improvement in the two step oxidation preparation of methacrylic acid.